1. Field of Invention
The present invention relates to a liquid crystal device and electronic apparatus. More particularly, the invention relates to an arrangement for a transflective liquid crystal device that includes a transflective film, having light transmitting opening portions, that also serves as pixel electrodes in the transmission mode.
2. Description of the Related Art
A reflective liquid crystal display consumes less power because it is not provided with a light source, such as a backlight, and has been used frequently in a display unit or the like as an attachment of various kinds of portable electronic apparatus or devices.
However, since outside light, such as natural light or illumination light, is used for a display, there is a problem that it is difficult to visually recognize a display in a dark place. Hence, there has been proposed a liquid crystal display that uses outside light in a bright place, like an ordinary reflective liquid crystal display, but that enables visual recognition of a display by an internal light source in a dark place. In other words, this liquid crystal display adopts a display method with both the reflection type and the transmission type, so that the display method is switched between the reflection mode and the transmission mode depending on the brightness of the surroundings, thereby enabling a sharp display even in dark surroundings, while also reducing power consumption. Hereinafter, the liquid crystal display of this type is referred to as a xe2x80x9ctransflective liquid crystal displayxe2x80x9d in the present specification.
As a type of the transflective liquid crystal display, the one provided with a transflective film, that is, a so-called xe2x80x9chalf mirror,xe2x80x9d has been known. In general, the transflective film is a film arranged so as to transmit light to some extent, while reflecting light to some extent by optimizing a film thickness of a film of metal, such as aluminum, used as a reflective film. However, there is a drawback in that a film forming technique, such as mask sputtering, is required to form a transflective film. The process also becomes more complicated, and the transmittance and the reflectance vary significantly due to considerable variance in film thickness.
Hence, in order to overcome the drawback of the transflective film, there has been proposed a liquid crystal display that is provided with a transflective film having light transmitting slits (opening portions) formed therein. When this liquid crystal display is used in the reflection mode in a bright place, outside light that is incident from above the top substrate passes through the liquid crystal layer, and is reflected at the portions other than the slits on the surface of the transflective film on the bottom substrate. Then, the light passes through the liquid crystal layer again, and travels out to the top substrate side. When the liquid crystal display is used in the transmission mode in a dark place, light emitted from a backlight that is provided below the bottom substrate passes through the slit portions of the transflective film. Then, the light passes through the liquid crystal layer and travels out to the top substrate side. These beams of light contribute to a display in each mode. The transflective film may be provided independently on the bottom substrate. However, because it is usually made of a film of metal, such as aluminum, it may also serve as pixel electrodes to drive liquid crystals. If the transflective film is used as such, an arrangement, such as the transflective liquid crystal display, becomes simpler.
Incidentally, in both cases of a passive matrix type and an active matrix type, the liquid crystal display is provided with a plurality of electrodes (which are generally referred to as xe2x80x9csegment electrodesxe2x80x9d in the passive matrix type, and as xe2x80x9cpixel electrodesxe2x80x9d in the active matrix type) to drive liquid crystals, which are arranged with a spacing of some micrometers on the bottom substrate. The liquid crystal molecules on the electrodes, especially those positioned at the center of the electrodes, are driven properly by a typical longitudinal electric field developed between the electrodes on the top and bottom substrates. However, the liquid crystal molecules disposed in a space between the electrodes, or in the vicinity of the edge portions of the electrodes, are susceptible to a lateral electric field developed between the adjacent electrodes, and a disturbance of the alignment (disclination) occurs. In particular, in case a reverse driving method to supply image signals of different polarities to the respective adjacent electrodes is adopted, the disclination caused by the lateral electric field appears noticeably. As a result, leakage of light occurs where it should be a black display, or conversely, the brightness is lowered when it should be a white display, and the contrast is deteriorated markedly. Hence, a countermeasure is taken to cover a disclination occurring area with a light-shielding film (black matrix). In such a case, however, there is a problem that the aperture ratio within the pixel is reduced, thereby making the display dark.
Also, the transflective liquid crystal display may adopt the arrangement that the electrodes, provided with slits on a one-by-one basis, also serve as the transflective film as discussed above. In such a case, like in the case of the liquid crystal molecules disposed in a space between the electrodes, the liquid crystal molecules above the slit are also susceptible to a lateral electric field that is developed from the surrounding electrodes, and the disclination occurs. Accordingly, a contrast-reduced area appears on the slit portion, which becomes a factor that further deteriorates the display quality. Moreover, such an occurrence of disclination also becomes an impediment to high definition.
The present invention addresses the above problems, and provides a liquid crystal device that is capable of obtaining a bright image with a high display quality by controlling disclination caused by a lateral electric field developed across a space between the pixels, or in the vicinity of the slit of the electrode in the device of the transflective type.
A liquid crystal device of the present invention includes a liquid crystal layer sandwiched by a pair of substrates, and at least one substrate of the pair of substrates is provided thereon with a plurality of electrodes to drive the liquid crystal layer. Where a twist angle xcex8 of the liquid crystal layer sandwiched by the pair of substrates is in a range of xcex8xe2x89xa690xc2x0, and d is a layer thickness of the liquid crystal layer, then alignment treatment in a direction corresponding to the twist angle is applied onto each of the pair of substrates in such a manner that, in a state that no voltage is applied, a direction of the long axis of a liquid crystal molecule positioned (xc2xc)d high from a surface of the one substrate is oriented to a direction that is perpendicular to a direction in which a space between the electrodes adjacent to each other extends.
Also, in the liquid crystal device, where a twist angle xcex8 of the liquid crystal layer sandwiched by the pair of substrates is in a range of 180xc2x0xe2x89xa6xcex8xe2x89xa6270xc2x0, and d is a layer thickness of the liquid crystal layer, then alignment treatment in a direction corresponding to the twist angle is applied onto each of the pair of substrates in such a manner that, in a state that no voltage is applied, a direction of the long axis of a liquid crystal molecule positioned (xc2xc)d high from a surface of the one substrate is oriented to a direction that is in parallel to a direction in which a space between the electrodes adjacent to each other extends.
The liquid crystal device having the above two features controls the disclination occurring chiefly between the pixels or at the periphery portions of the pixels, and is suited as a liquid crystal device to supply the respective adjacent pixels with image signals of different polarities, that is, the one adopting reverse driving as a driving method.
In addition, a liquid crystal device of the present invention that controls the disclination at an opening portion (slit) is a liquid crystal device includes a liquid crystal layer sandwiched by a pair of substrates, and at least one substrate of the pair of substrates is provided thereon with a plurality of electrodes that also serve as a transflective layer to reflect incident light from the other substrate side through the liquid crystal layer and transmit incident light from an outer surface side of the one substrate to the liquid crystal layer side through an opening portion, that is, a transflective liquid crystal display. Where a twist angle xcex8 of the liquid crystal layer sandwiched by the pair of substrates is in a range of xcex8xe2x89xa690xc2x0, and d is a layer thickness of the liquid crystal layer, then alignment treatment in a direction corresponding to the twist angle is applied onto each of the pair of substrates in such a manner that, in a state that no voltage is applied, a direction of the long axis of a liquid crystal molecule positioned (xc2xc)d high from a surface of the one substrate is oriented to a direction that is perpendicular to a direction in which the opening portion extends.
Also, in the liquid crystal device, where a twist angle xcex8 of the liquid crystal layer that is sandwiched by the pair of substrates is in a range of 180xc2x0xe2x89xa6xcex8xe2x89xa6270xc2x0, and d is a layer thickness of the liquid crystal layer, then alignment treatment in a direction corresponding to the twist angle is applied onto each of the pair of substrates in such a manner that, in a state that no voltage is applied, a direction of the long axis of a liquid crystal molecule that is positioned (xc2xc)d high from a surface of the one substrate is oriented to a direction that is in parallel to a direction in which the opening portion extends.
As discussed, for liquid crystal devices in general, including a transflective liquid crystal device, the occurrence of disclination caused by a lateral electric field that is developed when a voltage is applied poses a significant problem. On the other hand, when the device is designed, it is necessary to set a bright view direction, which is defined as a direction along which the liquid crystal molecules rise up when a voltage is applied and a display is viewed most brightly, in such a manner that the occurrence of the disclination is reduced to the minimum. The disclination occurring circumstances vary depending on various parameters, such as the twist angle of the liquid crystal molecules, an opening width, and an applied voltage, and for this reason, the optimal bright view direction is conventionally found by performing an experiment each time the parameters change. However, it has been a great burden to find the bright view direction experimentally each time the device condition changes.
Hence, the inventor of the present invention examined, through simulations, which parameter of various device parameters exerts a significant influence to the bright view direction, that is, the occurrence of the disclination. As a result, the inventor discovered that the initial alignment direction (the alignment direction when no voltage is applied) of the liquid crystals with respect to a direction of a space between the pixels, or the opening portion, is deeply involved. Also, the inventor discovered there is a tendency that the optimal angular relation between the direction of the space between the pixels or the opening portion and the initial alignment direction of the liquid crystals differs in an area where the twist angle xcex8 of the liquid crystal layer is xcex8xe2x89xa690xc2x0, and that is deemed as an area where a change of the twist angle of the liquid crystal molecules is larger than a change of the tilt angle when a voltage is applied, and in an area where the twist angle xcex8 of the liquid crystal layer is 180xc2x0xe2x89xa6xcex8xe2x89xa6270xc2x0, and that is deemed as an area where a change of the tilt angle of the liquid crystal molecules is larger than a change of the twist angle when a voltage is applied.
In other words, decrease of aperture ratio due to disclination can be reduced or minimized by applying the alignment treatment onto each substrate in such a manner that the major axial direction of the liquid crystal molecule positioned (xc2xc)d (d: layer thickness of the liquid crystal layer) high from the surface of one substrate is oriented to a direction that is perpendicular to the extending direction of the space between the pixels, or the opening portion, when the twist angle xcex8 is in a range of xcex8xe2x89xa690xc2x0, and by applying the alignment treatment onto each substrate in such a manner that the major axial direction of the liquid crystal molecule positioned (xc2xc)d high from the surface of one substrate is oriented to a direction that is in parallel to the extending direction of the space between the pixels, or the opening portion, when the twist angle xcex8 is in a range of 180xc2x0xe2x89xa6xcex8xe2x89xa6270xc2x0. These characteristics of the present invention are introduced from the simulation results, and the detailed description will be given below. xe2x80x9cOne substratexe2x80x9d referred to herein is a substrate that is provided with a plurality of electrodes supplied with image signals, and is generally the bottom substrate.
An electronic apparatus of the present invention is provided with the liquid crystal device of the present invention. According to this arrangement, it is possible to achieve an electronic apparatus that is provided with a liquid crystal display unit that realizes a bright image with a high display quality.